IWMI report on Glaciers and Snow cover in Himalayas in Changing Climate: Significant Impact on Seasonal flow of the Rivers in India

International Water Management Institute (IWMI) has recently published a study named Glacier Systems and Seasonal Snow Cover in Six Major Asian River Basins: Hydrological Role under Changing Climate, authored by Oxana S. Savoskul and Vladimir Smakhtin which claims that the hydrological role of the melt-water resources in six major rivers e.g. Indus, Ganges, Brahmaputra, Syr Darya, Amu Darya and Mekong of the Hindukush-Himalayan region (HKH) has been comprehensively assessed for the first time on a basin scale. Reviewing already published studies, this report draws some interesting conclusions regarding the role of glacier and snow meting for six river basins which includes three major rivers basins of India.

The map below shows area of the river basins included in this study. In this report, the term ‘melt-water resources’ has been used to cover glacier systems and seasonal snow cover. This report uses 1961-1990 status of melt-water resources as the baseline and compares with the 2001-2010 using the following characteristics: specific glacier runoff (average depth of annual discharge from glacier-covered area), basin total glacier runoff, shares of renewable and nonrenewable components in glacier runoff, total seasonal surface snowmelt from non-glaciated areas, portion of seasonal snowmelt lost for the recharge of groundwater aquifers, the contribution of glacier runoff and seasonal snowmelt to mean annual flow (MAF).

Map from the report showing the boundaries of the study basins (red line), state borders (light yellow line) and snow-covered high-altitude belts where glaciers are located (white spots)

The authors have used Glacier mass budget-based methods and hydrograph separation techniques which they stated as suitable for basin-scale assessments instead of the temperature-index methods. They say that application of these two methods in semi-distributed models can give the highest currently possible accuracy of +30%. The authors opine that many of the studies done previously had confused between the ‘snowmelt’ and ‘glacier-melt’ because these studies have not dealt with terminologies and methodologies in detail. The report states that there is a scarcity of glacier runoff estimates in peer-reviewed papers, “An analysis of publications on modeling runoff from large- and medium-scale glaciated catchments….. indicates that not many of these dealt with modeling glacier runoff per se. Even fewer report their evaluations of glacier runoff separately from snowmelt, if at all.”

For the three of the six river basins studies and which flow through India, i.e. Indus, Ganges and Brahmaputra the total annual glacier runoff for the period of 1961-1990 was 41 km3,16 km3 and 17 km3 respectively. But in the recent periods of 2001-2010, total glacier runoff was reduced to 36 km3, 15 km3 and 16 km3 respectively for the three basins, see Table 1 for details.

It is clear from the table that while Indus and Brahmaputra basins have similar percentage of area under glaciers and snowmelt, the reduction in the glacier and snow cover area are more pronounced in Indus basin. Besides, in all the three basins the reduction in glacier area is more pronounced that the snow cover area. However, the contribution of glacier melt and also snow melt to run-off is much higher in Indus basin compared to Brahmaputra basin, showing the greater role of precipitation in Brahmaputra basin. Within the Indus basin even though seasonal snow covers 28% of the total area, much than the 2.6% occupied by glaciers during 1961-90, the contribution of two sources to Mean Annual Flow is almost same. But a question arises, has the contribution of glacier melt to the runoff increased in any of the basins in the recent decade? The answer is surprisingly, no.

Table 1: Recent changes in the glaciers and seasonal snow and their contributions to MAF

Basin

Part of basin area(%) covered by

Contribution to MAF (%)

Glaciers

Seasonal Snow

Glacier runoff

Seasonal Snowmelt

1961-1990

INDUS

2.6

28

18

19

GANGES

1.2

6

4

2

BRAHMAPUTRA

2.7

27

2

2

2001 -2010

INDUS

1.8

25

15

16

GANGES

0.9

6

3

1

BRAHMAPUTRA

2.2

26

2

2

For the Ganges basin, the report says that heavy summer precipitation almost solely determines MAF volume for the basin. Maximum seasonal snow area in the Ganges basin makes just 6% of the entire basin area. Similar situation were reported for the Brahmaputra basin, where the lower parts of the basin i.e. Southeastern Tibet and Eastern Himalayas where nearly 75% of the basin’s glaciers are located, witness heavy summer monsoon rains. Regarding Indus basin the report says, “Precipitation in the IndusBasin is more evenly distributed between the seasons, but is highly variable spatially – similar to Brahmaputra and Amu Darya, where annual precipitation in some catchments is tenfold (3,000 mm) of that in the other glacier-covered parts of the basin (300 mm).”

Reviewing already published documents the report states “it appears that the research in High Asia is concerned much more with CC impacts than with objects of the impact. Yet, understanding of the expected basin-scale changes in glacier runoff in response to climate change remains largely unclear.”

The report does an analysis of assessments done on impact of climate change on water availability in Himalayas and concludes that many assessments rely on poorly verified sources. The report refers to the statement made by Cruz et al. (2007) “The current trends of glacier-melts suggest that the Ganga, Indus, Brahmaputra and other rivers that criss-cross the northern Indian plain could likely become seasonal rivers in the near future as a consequence of climate change…,” This statement was admitted as a typing error after publication but even then this has been reiterated as an apocalyptic vision in NGO reports.

Using the Table 2 given below, the report states that glacier contribution is a minor item in the annual river water budgets in the Ganges and Brahmaputra basins. The report says “The impact of climate change was found to be more prominent on seasonal rather than annual water availability.” It is clear from the table that, in the recent decades non-renewable component in all three basins have gone up while renewable and total volume of water from glacier melt have come down. It is also noteworthy that, even though Brahmaputra basin has more area under glacier cover than the Ganges basin (see Table 1), the volume of water from non renewable glacier flow was more in both periods in the Ganges basin. Besides, the percentage of increase in nonrenewable glacier runoff components during 2001-10 is highest among all three basins, signifying that glaciers are melting fastest in Ganga basin.

The reports also states, “Glaciers and seasonal snow in CC-impact assessments should be perceived as natural water reservoirs with gradually diminishing storage and flow regulation capacity, both on intra-annual and inter-annual scale. Potential changes of precipitation regime coupled with effects of temperature rise on evapo-transpiration will impact future hydrological regimes of the major rivers much more significantly, affecting both MAF and flow seasonality.”

The authors of this report clear some fog around climate change and Himalayan glacier system and snow-melt. One lacuna of the report is that even though the report discusses glacier run-off it makes no mentions of glacier lakes and glacier lakes induced floods. There are several incidents of glacier lake induced floods happening in the basins discussed. There is evidence to show that in the recent flood devastation in Uttarakhand in India glacial lakes played significant role.